Halftone color separations are used in reprography as copy originals for making offset and letterpress printing plates. The color separations are checked prior to exposing with the aid of color proofing processes to determine whether the ultimate printing result will be a tonally correct reproduction of the original.
Such color proofing processes use photosensitive recording materials whereby upon exposure to actinic radiation, an image is produced due to differences in the tackiness of the exposed and unexposed areas of the photosensitive coating. Colored toners are then applied to the tacky areas after imagewise exposure.
The toners are predominantly finely divided powders, and are applied by dusting onto the imagewise exposed surface. These toners are disclosed, for example, in U.S. Pat. No. 4,215,193 issued Jul. 29, 1980; and U.S. Pat. No. 3,909,282 issued Sep. 30, 1975. Alternatively, the toner can be loosely bonded to a separate support to form a coated transfer material which is brought into contact with the imagewise exposed layer. Such transfer materials are disclosed in U.S. Pat. No. 3,060,025 issued Oct. 23, 1962; German Patent 29 49 462-C published Jun. 19, 1980; U.S. Pat. No. 4,806,451 issued Feb. 21, 1989; U.S. Pat. No. 4,902,363 issued Feb. 20, 1990; U.S. Pat. No. 4,939,029 issued Jul. 3, 1990; U.S. Pat. No. 5,126,226 issued Jun. 30, 1992; German Patent 39 41 493-C published May 2, 1991; and U.S. Pat. No. 4,935,331 issued Jun. 19, 1990.
Reproduction processes suitable for making positive color images of exposed originals are disclosed in German Patent 12 10 321-C (U.S. Pat. No. 3,620,726, issued Nov. 16, 1971); U.S. Pat. No. 3,582,327 issued Jun. 1, 1971; U.S. Pat. No. 3,649,268 issued Mar. 14, 1972; U.S. Pat. No. 4,356,253 issued Oct. 26, 1982; U.S. Pat. No. 4,892,802 issued Jan. 9, 1990; and U.S. Pat. No. 4,948,704 issued Aug. 14, 1990. In these processes, a tacky, photopolymerizable recording material, comprising a support and a photopolymerizable coating containing at least one addition-polymerizable monomer and a photopolymerization initiator, is hardened by imagewise exposure whereby the exposed areas lose their tackiness. The latent image is then made visible by applying a toner that adheres only to the unexposed, tacky areas. Excess toner remaining on the exposed, non-tacky image areas is removed after application.
Negative images of exposed originals are obtained with such photopolymerizable materials according to the process disclosed in U.S. Pat. No. 4,869,996 issued Sep. 26, 1989, wherein the tacky, unexposed areas of the imagewise exposed material are covered with a colorless powder. The entire material is then heated until the exposed, photopolymerized areas become tacky and can be colored with a toner. However, this process requires two additional process steps.
Negative images of the original can be made directly by using the photosensitive materials disclosed in U.S. Pat. No. 4,243,741 issued Jan. 6, 1981; U.S. Pat. No. 4,346,162 issued Aug. 24, 1982; U.S. Pat. No. 4,358,529 issued Nov. 9, 1982; U.S. Pat. No. 4,604,340 issued Aug. 5, 1986; U.S. Pat. No. 4,294,909 issued Oct. 13, 1981; and U.S. Pat. No. 4,356,252 issued Oct. 26, 1982. The tackiness of these materials is increased by irradiation with actinic radiation. These recording materials contain either a dihydropyridine compound or a photosensitive system of a hexaaryl-bisimidazole compound and a dihydropyridine compound or a thioamide or thiolactam. Hexaarylbisimidazoles are also disclosed in combinations with leuco dyes in German Patent C 13 00 013. The advantage of using materials that become tacky on exposure is that several colors can be applied together by repeated exposure through different originals and subsequent toning with different toners.
U.S. Pat. No. 4,876,172 issued Oct. 24, 1989 discloses a process in which microparticle layers that become tacky on exposure can also be toned, after imagewise exposure, by being brought into contact with a receptor so that only the exposed areas are transferred.
U.S. Pat. No. 4,461,822 issued Jul. 24, 1984 describes a process to increase the density or modify the color of imagewise toned surfaces by multiple applications of different powdered toners that become tacky on contact with each other.
However, no process is known for making toned polychromatic negative images with overlapping areas of different colors on a single photosensitive recording layer. Making such polychromatic images requires application of a new photosensitive layer onto the already toned layers, followed by imagewise exposure, and a subsequent toning step. The color overlap required for the resulting proof print cannot be achieved otherwise. Also, exact registration of the individual monochromatic images with each other is particularly difficult. In addition, such multi-step processes are very time-consuming and can lead to color inaccuracies because the photosensitive layer for each color remains within the proof print.
Accordingly, the present invention enables one to make polychromatic images, especially negative color proof prints, with overlapping color areas without the need to transfer exposed image areas or the addition of other intermediate steps.